1. Field of the Invention
This invention relates to open top vessels such as tanks, bins and hoppers, other than round tanks, which may either stationary or as part of trucks, trailers, ocean shipping containers or other mobile conveyance, for storing, transporting or processing bulk materials and liquids. More specifically, this invention relates to vessels comprised of fiber reinforced plastic (FRP) wherein the high tension carrying capabilities of FRP are utilized to support and contain the bulk materials or liquids by means of a thin flexible FRP shell, supported along its upper sides and/or top edge by a frame consisting of FRP, steel, aluminum, or other materials. Even more specifically, the instant invention relates to a hopper bottom trailer wherein the hopper bottoms are constructed of thin, flexible FRP shells.
2. Description of the Related Art
The conventional method of constructing large open top vessels, other than round vessels, is to construct the shell or box consisting of flat sides and bottom in the form of a box, cone, trapezoid, or other shape necessary to contain and/or discharge the contained material or accomplish other intended purposes. The loads, lateral and vertical forces, exerted by the contained material on the flat walls and bottom of the vessel tend to cause the flat bottom and walls of the vessel to deflect and become convex in shape. If the loading is significant, the walls and bottom may buckle or rupture. To prevent buckling or rupturing, the walls and bottom of the vessel are constructed so as to be thick and/or rigid or are further reinforced with ribs, support beams or other supporting structure capable of resisting the forces applied by the material contained in the vessel. In either event, the walls and bottom and any supporting beams or ribs are subject to bending. When such vessels or containers are constructed of steel, the resultant structure requires significant labor to fabricate, is heavy, and is subject to corrosion. The weight of a steel structure is a disadvantage when being used on a truck, trailer or other mobile conveyance. When the vessel is made of aluminum, the structure also requires significant labor to fabricate and if the aluminum sheets are bolted or riveted together, seams, crevices and other protrusions are formed in the bottom and walls of the vessel that may leak, disrupt or impede the smooth flow of material from the vessel when it is used as a hopper or process vessel. When such vessels are made of FRP, which is lighter than steel or aluminum, but which often has a lower modulus of elasticity in bending than steel or other conventional material, the walls of the vessel must be made thicker, or the ribs or reinforcing beams must be made thicker and placed closer together to resist the bending forces to prevent unacceptable deflection.
The conventional method of manufacturing large open top vessels from FRP has the disadvantage of requiring thicker material to resist bending and the joining of many parts which requires substantial labor. Since FRP materials are usually more expensive than steel, the cost of manufacturing FRP vessels places them at a cost disadvantage with respect to steel and other conventional materials. On the other hand, FRP is more resistant to corrosion than steel and other conventional materials; and would be the material of choice for lightweight vessels and those containing liquids or corrosive materials if the disadvantages associated with its low modulus of elasticity could be overcome to reduce the amount of material required to lower the cost of producing such vessels.
Generally speaking, conventional hopper bottom trailers include one or more metal hopper bottoms having discharge openings at the lower ends thereof with the discharge openings normally being positioned between the wheels of the trailer. Normally, side walls extend upwardly from the hopper bottom to contain the material being transported and to direct the material being transported to the hopper bottom for discharge therefrom. Usually, the hopper bottoms are constructed of a steel or aluminum material with the hopper bottoms normally being constructed of sheets of metal which are bolted, welded or riveted together to form the cone-shaped hopper bottom which is necessary to direct the material being transported to the discharge opening. The construction of the hopper bottoms is time-consuming due to the need for connecting the various panels or segments together to form the hopper bottom.